Recently, for enhancing safety of a driver who drives a vehicle at night (or at the time of low illumination), a technique of mounting a night vision apparatus on a vehicle, and displaying for a driver an image (a night vision image) of a road ahead the vehicle with high visibility has been put into practical use. The night vision apparatus of a vehicle mounting type is classified roughly into a night vision apparatus using a far-infrared system and a night vision apparatus using a near-infrared system.
The night vision apparatus using a far-infrared system realizes its function by imaging heat (far-infrared ray) generated by a person or a vehicle on the road ahead by a far-infrared camera. Because a heat source itself is visualized, imaging is possible even at a circumstance with no lighting. However, in the far-infrared system, imaging is not possible when a background and an object have no difference in temperature, so that road signs, road surface paint, or the like cannot be imaged. Furthermore, the night vision apparatus using a far-infrared system is expensive.
On the other hand, the night vision apparatus using a near-infrared system emits near-infrared light ahead, images the reflected light with a camera that can image a near-infrared wavelength band, and displays the image. The system can image even an object that does not generate heat, and can obtain an image similar to a visual image. Furthermore, the night vision apparatus using a near-infrared system is less expensive than the night vision apparatus using a far-infrared system.
However, the night vision apparatus using a near-infrared system is susceptible to an illuminant with strong light such as a headlight of an opposing vehicle, and cannot read road signs made of a recursive reflection material due to strong reflection.
To solve the problems, a technique of reducing influence of an illuminant by emitting near-infrared light in pulses and correcting a night vision image by using a difference image between an image when lighting and an image when not lighting (see Japanese Laid-open Patent Publication No. 2002-274258 and Japanese Laid-open Patent Publication No. 2003-87644).
However, there has been a problem that even when a night vision image is taken by using a night vision apparatus using a near-infrared system, and the night vision image is corrected by using the conventional techniques (for example, Japanese Laid-open Patent Publication No. 2002-274258 or Japanese Laid-open Patent Publication No. 2003-87644), visibility of an object having certain characteristics (for example, an object made of a recursive reflection material (road signs or the like)) cannot be enhanced.